The present invention is an improvement on applicant's prior U.S. Pat. No. 8,251,139, issued Aug. 28, 2012, and entitled SYSTEM AND METHOD FOR SEALING COUPLINGS IN DOWNHOLE TUBING STRINGS. The disclosure of this prior patent is expressly incorporated herein by reference in its entirety.
Water injection of oil and gas wells, to increase the production efficiency of these wells, has become quite well-known and widely practiced in the oil and gas production industry. In this procedure, water under pressure is injected down an injection well, which injection well is comprised of a large number of tubing or casing sections coupled to each other by coupling sleeves. The injection of such water under pressure has been found to greatly improve the recovery of oil from oil fields and is a generally accepted production process. Increases in well production can also be obtained through the injection of a recovery enhancement fluid into production wells for short periods, and after a soak period, by putting the well back into production.
Applicant's prior patent is directed to a seal ring and coupling sleeve assembly which allows tubing strings to be repeatedly assembled and disassembled without the need to replace individual coupling sleeves and/or seal rings. The prior assembly has enjoyed large commercial acceptance in the industry, primarily because of its ability not to collapse or to blow out or to experience explosive decompression when tubing strings are depressurized or bled down during removal of a tubing string and the disassembly of the tubing string into its respective tubular sections and coupling sleeves. The prior assembly, as described and depicted in applicant's U.S. Pat. No. 8,251,139, utilizes a Teflon® seal ring with a polyether ether ketone or PEEK reinforcing ring, together with a circumferential gas groove and two opposed, radially directed gas ports that bleed off any pressure which may have built up between the outer periphery of the annular seal ring and the inner periphery of the coupling sleeve. As noted above, these seal ring and coupling sleeve assemblies have been widely accepted and used in the water injection production of oil and gas from underground wells. Seal ring failures, due to a build-up of gas pressure, during tubing string disassembly have been virtually non-existent.
The oil and gas production industry has begun utilizing water alternating gas injection or WAG procedures to improve the volumetric sweep efficiency of miscible flooding pressures. In a WAG injection process, water at high pressure is injected, through an injection well, into a particular field, typically for a period of twenty-four hours. This water injection is then stopped and is replaced by the injection of CO2 gas, typically at a correspondingly high pressure, for a period of 30 to 40 hours or more.
Increased utilization of EOR or enhanced oil recovery systems, such as WAG injection, are becoming more commonplace in the oil and gas industry. In such systems, pressures of water and CO2 as high as 4,000 psi or greater are now being used to greatly increase oil recovery. In reef-based limestone formations, the use of alternating water and CO2 or WAG injection procedures, may dissolve the limestone formations. The alternating water and CO2 injection procedure also causes the entrapped oil to become thinner and to flow much more freely. In most instances, dramatic increases in well production have been experienced using EOR systems, such as WAG.
WAG systems are not limited to the use of water alternating with CO2 gas. In some geological formations, the use of CO2 gas may actually have a detrimental effect on oil recovery. For example, in some shale formations, the use of CO2 gas may cause the formations to swell, thereby reducing production. In such situations, natural gas, CH4 may be used as an alternative to CO2.
While this WAG injection process has increased oil field production and recovery, it has also resulted in a failure rate of approximately 12%-15% of applicant's prior seal ring and coupling sleeve assemblies, as disclosed in the aforementioned U.S. Pat. No. 8,251,139. Such seal failure manifests itself as an explosive decompression which may occur during the bleeding down of a well tubing assembly.
One result of the use of WAG injection is the formation of carbonic acid H2CO3. The formation of this acid is the result of the injection of CO2 gas into a well string which is still wet from the prior use of water under pressure. Although the formation of carbonic acid in tubing and casing sections is attempted to be kept at a minimum by the alternating use of water injection and then CO2 gas injection, the formation of such carbonic acid in the tubing and casing strings is an unavoidable result. The formation of such carbonic acid may have detrimental effects on seal rings and can exacerbate seal ring failures.
Carbon dioxide gas has been determined to permeate typical seal ring materials, such as Teflon®. Over time, the use of WAG injection and similar EOR production techniques results in a buildup of CO2 or other gas under pressure in an area between the outer circumferential surface of a seal ring and the inner circumferential surface of the overlying coupling sleeve. Explosive decompression is the phenomenon that occurs during the bleeding down of wells in which the tubing or casing has been subjected to WAG or other EOR system procedures. In such explosive decompression situations, the pressure which has built up between the seal ring outer circumference and its associated coupling sleeve inner surface, when the pressure in the tubing or casing section is being bled down, will not be released gradually into the interior of the tubing. Instead, it will rapidly and violently deform and distort the resilient material which forms the seal ring assembly, particularly on the field side of the seal ring and coupling sleeve assembly. If the built-up pressure is not properly reduced, by venting into the interior of the tubing pipe string, when the well is depressurized, the result may be such an explosive decompression in the field end, of ones of the coupling sleeves when the individual tubing sections are disconnected from each other. Such a failure of the seal ring assembly in a coupling sleeve, due to such explosive decompression, will render that seal ring and coupling sleeve unsuitable for reuse. The resilient seal ring assembly material, once it has been distorted, will not return to its original shape. Applicant has determined that this inability to properly vent or to release built-up CO2 or other types of gas pressure, particularly in WAG injection field environments, has led to the 12-15% seal failure rate discussed above. This seal failure rate is deemed to be unacceptable and has been the impetus for the development and implementation of the seal ring and coupling sleeve assembly to be described hereinafter, which seal ring and coupling sleeve assembly overcomes the limitations of the prior art.